Nanoparticles with Multiple Attached Polymer Assemblies and Use Thereof in Polymer Composites

ABSTRACT

Methods of synthesizing a binary polymer functionalized nanoparticle are generally provided. In one embodiment, a first anchoring compound is attached to a nanoparticle, and a first plurality of first monomers are polymerized on the first anchoring compound to form a first polymeric chain covalently bonded to the nanoparticle via the first anchoring compound. In another embodiment, a first polymeric chain can be attached to the nanoparticle, where the first polymeric chain has been polymerized prior to attachment to the nanoparticle. Thereafter, a second anchoring compound is attached to the nanoparticle, and a second plurality of second monomers are polymerized on the second anchoring compound to form a second polymeric chain covalently bonded to the nanoparticle via the second anchoring compound. Nanoparticles are also generally provided having multiple polymeric assemblies.

PRIORITY INFORMATION

The present application is a divisional of and claims priority to U.S.patent application Ser. No. 14/519,223 titled “Nanoparticles withMultiple Attached Polymer Assemblies and Use Thereof in PolymerComposites” of Benicewicz, et al. filed on Oct. 21, 2014, which is adivisional of and claims priority to U.S. patent application Ser. No.13/569,780 titled “Nanoparticles with Multiple Attached PolymerAssemblies and Use Thereof in Polymer Composites” of Benicewicz, et al.filed on Aug. 8, 2012, and claims priority to U.S. Provisional PatentApplication Ser. No. 61/521,594 titled “Nanoparticles with MultipleAttached Polymer Assemblies and Use Thereof in Polymer Composites” ofBenicewicz, et al. filed on Aug. 9, 2011; the disclosures of which areincorporated by reference herein.

BACKGROUND

Controlled radical polymerization has been widely used to synthesizedifferent polymer architectures such as block copolymers, star polymers,gradient copolymers and polymer brushes. One of the interestingapplications is the synthesis of polymer brushes on nanoparticles forincorporation into polymer nanocomposites. In polymer nanocompositescience, it has been well established in the recent years that chemicalgrafting of a homopolymer having the same chemistry as that of thepolymer matrix onto the inorganic nanoparticle substrate shields thenanoparticle from the matrix. Theoretically this should prevent thematrix from de-wetting the substrate. However, in several instancesthere still remains an unfavorable entropic interaction between thegrafted chains and the matrix, which causes the system to de-wet. Thisphenomenon is known as autophobic de-wetting and is described in detailby many researchers and is now well understood. Through simulation,Matsen et al. have demonstrated that the key to suppressing autophobicde-wetting lies at broadening the brush polymer/matrix polymer interfacewhich can be achieved by using a bimodal system that contains a smallnumber of long homopolymer chains in a brush which primarily consists ofshort dense brushes.

Although single component monodisperse polymer brushes have beensuccessfully grafted from a variety of substrates including silicananoparticles using a wide variety of techniques, there are surprisinglyvery few methods in the literature describing the synthesis of mixedpolymer brush grafted surfaces. The first synthesis of mixed brushes wascarried out independently by Minko et al.⁴ and Dyer et al.⁵ using the‘grafting from’ approach based on surface anchored azo initiators. Thesynthesis of mixed brushes using controlled radical polymerization hasbeen demonstrated by Zhao et al. who used a two-step ATRP and NMPreaction to graft PMMA and PS from a silica surface. However, thesemethods utilized a split anchoring agent (i.e., a “Y” anchoring agentwith two functionalized groups extending therefrom). Then, eachfunctionalized group can be used to attach a polymeric chain thereto.

As such, these methods are limited in the types of monomers required forseparate polymerization. Additionally, these methods necessarily form aparticle with a 50/50 percentage of each polymeric chain.

Thus, a need exists for improved methods for synthesizing nanoparticleswith multiple polymer assemblies attached.

SUMMARY

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

Methods are generally provided for synthesizing a binary polymerfunctionalized nanoparticle. In one embodiment, a first anchoringcompound is attached to a nanoparticle, and a first plurality of firstmonomers are polymerized on the first anchoring compound to form a firstpolymeric chain covalently bonded to the nanoparticle via the firstanchoring compound. In another embodiment, a first polymeric chain canbe attached to the nanoparticle, where the first polymeric chain hasbeen polymerized prior to attachment to the nanoparticle.

Thereafter, a second anchoring compound is attached to the nanoparticle,and a second plurality of second monomers are polymerized on the secondanchoring compound to form a second polymeric chain covalently bonded tothe nanoparticle via the second anchoring compound.

Nanoparticles are also generally provided having multiple polymericassemblies. For example, a first polymeric chain can be covalentlybonded to the surface of the nanoparticle; and a second polymeric chaincovalently bonded to the surface of the nanoparticle.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, which includesreference to the accompanying figures, in which:

FIG. 1 shows a synthetic scheme describing the synthesis of binarybrushes on silica nanoparticles using step-by-step RAFT polymerizationaccording to one exemplary embodiment described in detail in theExamples;

FIG. 2 shows an exemplary GPC trace of the bimodal distribution ofpolymer chains according to one example;

FIG. 3 shows a scheme for the immobilization of 4-cyanopentanoic aciddithiobenzoate (CPDB) as an exemplary first anchoring compound on thesurface of a Si nanoparticle;

FIG. 4 shows a scheme for deactivation and cleavage of CPDB chain endform Si-g-PMMA₁ using AIBN as an exemplary first polymeric chain;

FIG. 5 shows a scheme for synthesis of PMMA and PS grafted colloidalsilica nanoparticles as an exemplary first and second polymeric chain;and

FIG. 6 shows a scheme for ATRP reaction from the surface of Si-g-PS tosynthesize an exemplary second polymeric chain.

DETAILED DESCRIPTION

Reference now will be made to the embodiments of the invention, one ormore examples of which are set forth below. Each example is provided byway of an explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in the inventionwithout departing from the scope or spirit of the invention. Forinstance, features illustrated or described as one embodiment can beused on another embodiment to yield still a further embodiment. Thus, itis intended that the present invention cover such modifications andvariations as come within the scope of the appended claims and theirequivalents. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly, and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied exemplary constructions.

Versatile and widely applicable step-by-step methods are generallyprovided to synthesize nanoparticle with multiple polymeric assembliesthrough polymerization (e.g., RAFT polymerization). In one exemplarymethod, consecutive step-by-step polymerizations (e.g., utilizing RAFTpolymerization) can be used to prepare nanoparticle with multiplepolymeric assemblies. In another exemplary method, RAFT polymerizationfollowed by ATRP polymerization can be used to synthesize nanoparticlewith multiple polymeric assemblies.

Through these methods, nanoparticles can be functionalized with multiplepolymeric assemblies. In particular each nanoparticle can have at leasttwo different polymeric chains extending therefrom. In certainembodiments, the nanoparticle with multiple polymeric assemblies areformed while maintaining simultaneous control over multiple variables,including but not limited to monomer-type, grafted chain molecularweight, polydispersity, etc. The grafted polymer chains, which arecovalently attached to the nanoparticle surface, can help improve thedispersion of the nanoparticles in an organic polymer matrix.

In one embodiment, two different types of polymeric assemblies (e.g., afirst polymeric chain and a second polymeric chain) can be attached tothe nanoparticle. In other embodiments, a third type of polymericassembly (i.e., a third polymeric chain) can also be attached.Additional polymeric assemblies (e.g., a fourth polymeric chain) canalso be attached to the surface, depending on the available surface areaon the nanoparticle and/or the size, dispersity, and/or density of thefirst, second, and third polymeric chains already present on the surfaceof the nanoparticle.

Nanoparticles with multiple assemblies of polymer chains may be used inpolymer nanocomposites, that is, mixtures of nanoparticles with one ormore matrix polymers. Such composites may find use in composites withimproved mechanical properties, optically clear composites withdispersed nanoparticles for applications such as high refractive indexmaterials, enhance dielectric or conducting properties, or combinationsof such properties, known as multifunctional materials. Thenanoparticles may also have specific and targeted biological activity,such as antibacterial properties.

1. Nanoparticles:

The presently disclosed methods can be utilized on a variety ofdifferent types of nanoparticles. The nanoparticle may comprise, forexample, natural or synthetic nanoclays (including those made fromamorphous or structured clays), inorganic metal oxides (e.g., silica,alumina, and the like), nanolatexes, organic nanoparticles, etc.Particularly suitable nanoparticles include inorganic nanoparticles,such as silica, alumina, titania (TiO₂), indium tin oxide (ITO), CdSe,etc., or mixtures thereof. Suitable organic nanoparticles includepolymer nanoparticles, carbon, graphite, graphene, carbon nanotubes,virus nanoparticles, etc., or mixtures thereof

Nanoparticles as used herein means particles (including but not limitedto rod-shaped particles, disc-shaped particles, platelet-shapedparticles, tetrahedral-shaped particles), fibers, nanotubes, or anyother materials having at least one dimension on the nano scale. In oneembodiment, the nanoparticles have an average particle size of about 1to about 1000 nanometers, preferably 2 to about 750 nanometers. That is,the nanoparticles have a dimension (e.g., a diameter or length) of about1 to 1000 nm. Nanotubes can include structures up to 1 centimeter long,alternatively with a particle size from about 2 to about 50 nanometers.Due to their size, nanoparticles have very high surface-to-volumeratios.

The nanoparticles may be crystalline or amorphous. A single type ofnanoparticle may be used, or mixtures of different types ofnanoparticles may be used. If a mixture of nanoparticles is used theymay be homogeneously or non-homogeneously distributed in the compositematerial or a system or composition containing the composite material.Non-limiting examples of suitable particle size distributions ofnanoparticles are those within the range of about 2 nm to less thanabout 750 nm, alternatively from about 2 nm to less than about 200 nm,and alternatively from about 2 nm to less than about 150 nm.

It should also be understood that certain particle size distributionsmay be useful to provide certain benefits, and other ranges of particlesize distributions may be useful to provide other benefits (forinstance, color enhancement requires a different particle size rangethan the other properties). The average particle size of a batch ofnanoparticles may differ from the particle size distribution of thosenanoparticles. For example, a layered synthetic silicate can have anaverage particle size of about 25 nanometers while its particle sizedistribution can generally vary between about 10 nm to about 40 nm.

In one embodiment, the nanoparticles can be exfoliated from a startingmaterial to form the nanoparticles. Such starting material may have anaverage size of up to about 50 microns (50,000 nanometers). In anotherembodiment, the nanoparticles can be grown to the desired averageparticle size.

2. Attaching a First Anchoring Compound to the Nanoparticle:

In certain embodiments, a first anchoring compound can be attached tothe surface of the nanoparticle for subsequent attachment of the firstpolymeric chains (e.g., via a “grafting-from” or “grafting-to” approach,as described in greater detail below). The first anchoring compound iscovalently bonded to the surface of the nanoparticle, either directly orvia a first functionalization group. The particular anchor compound canbe selected based upon the type of nanoparticle and/or the type ofpolymeric chain to be attached thereto.

The first anchoring compound has a functional group for furtherreaction. Suitable functional groups for further reaction can include,but are not limited to, amine groups (e.g., amide groups, azide groups,cyanate groups; nitrate groups, nitrite groups, etc.), thiol groups(e.g., sulfinic acid, sulfonic acid, thiocyanates, etc.), phosphonategroups, hydroxyl groups (e.g., —OH), carboxylic acid groups (e.g.,—COOH), aldehyde groups (e.g., —CHO), halogen groups (e.g., haloalkanes,haloformyls, etc.), epoxy groups, alkenes, alkynes, and the like. Forexample, the anchoring compound can be a RAFT agent, when used with aRAFT polymerization technique.

For example, in one particular embodiment, 4-cyanopentanoic aciddithiobenzoate (CPDB) can be attached to the surface of the nanoparticleas a first anchor, as shown in FIG. 3. In this embodiment, thedithioester anchoring compound can be immobilized onto the surface ofthe nanoparticles (e.g., colloidal silica nanoparticles). For instance,the 4-cyanopentanoic acid dithiobenzoate (CPDB) anchoring compound canbe attached on the surface of the nanoparticles by first functionalizingthe surface of the nanoparticles with amine groups using3-aminopropyldimethylethoxysilane. Use of a mono-functional silane suchas 3-aminopropyldimethylethoxysilane compared to a trifunctional silaneensures the formation of a monolayer of initiator on the silica surfaceand prevents particle agglomeration by crosslinking during processing.The ratio of the 3-aminopropyldimethylethoxysilane to silicananoparticles is critical in determining the grafting density. Inaddition to adjusting the ratio by varying the concentration ofamino-silane, addition of a small amount of an inertdimethylmethoxy-n-octylsilane helps to partially cover the silicasurface by inert alkyl groups and helps to tune the grafting densityalong with preventing aggregation of the nanoparticles. To attach theanchoring compound onto the amine functional silica, the4-cyanopentanioc acid dithiobenzoate can be first activated by using2-mercaptothiazoline. It can then immobilized onto the surface of silicavia a condensation reaction with the amine groups on the silica surface.Using this approach, various CPDB-functionalized nanoparticles can besynthesized having a grafting density varying from 0.01-0.7 anchoringcompounds/nm². An inherent advantage of this technique compared to theother “grafting-from” methods is the ease and accuracy in measuring thegrafting density before carrying out the polymerization. The CPDBmolecule is UV-VIS active and hence by comparing the absorption at 302nm from the CPDB-functionalized nanoparticles to a standard absorptioncurve made from known amounts of free CPDB, the concentration of theanchoring compounds attached onto the nanoparticles can be calculated.Knowledge of the concentration of the anchoring compounds attached ontothe nanoparticles before the reaction provides the reaction with controland predictability, which is the key to controlling molecular weight andmolecular weight distribution.

3. Attaching a First Polymeric Chain to the First Anchoring Compound:

Two methods can be utilized to form the first polymeric chain extendingfrom the nanoparticles via the first anchoring compound: a“grafting-from” approach and a “grafting-to” approach. These strategieswill be explained in more details in the following sections.

A. “Grafting-from” Methods

In one embodiment, the first polymeric chain can be formed bypolymerizing a first plurality of first monomers on the first anchoringcompound, resulting in the first polymeric chain being covalently bondedto the nanoparticle via the first anchoring compound. According to thismethod, the polymerization of the first polymeric chain can be conductedthrough any suitable type of polymerization, such as RAFTpolymerization, ATRP, etc., which are discussed in greater detail below.

The particular types of monomer(s) and/or polymerization technique canbe selected based upon the desired polymeric chain to be formed. Forexample, for RAFT polymerization, monomers containing acrylate,methacrylate groups, acrylamides, styrenics, etc., are particularlysuitable for formation of the first polymeric chain.

Thus, the “grafting-from” method involves formation of the firstpolymeric chain onto the first anchoring compound and results in thefirst polymeric chain being covalently bonded to the nanoparticle viathe first anchoring compound (and, if present, a first functionalizationcompound).

B. “Grafting-to” Methods

In one embodiment, the first polymeric chain can be first polymerizedand subsequently covalently bonded to the surface of the nanoparticle,either directly or via a first anchoring compound (and, if present, afirst functionalization compound). Thus, in this embodiment, the firstpolymeric chain has been polymerized prior to attachment to the firstanchoring compound.

In this embodiment, the first polymeric chain is not limited to the typeof polymerization and/or types of monomer(s) capable of beingpolymerized directly to the first anchoring compound. As such, as longas the first polymeric chain defines a functional group that can reactand bond to the first anchoring compound, any polymeric chain can bebonded to the nanoparticle.

4. Deactivating the First Polymeric Chain:

No matter the method used to attach the first polymeric chain to firstanchoring compound on the nanoparticle, upon attachment, the firstpolymeric chain is, in one particular embodiment, deactivated to preventfurther polymerization thereon.

For example, if the “grafting-from” method was utilized to attach thefirst polymeric chain to the first anchoring compound via polymerizationthrough a CLP technique (e.g., RAFT), a deactivation agent can beattached to the end of each polymeric chain to inhibit furtherpolymerization thereon. The deactivation agents can be selected basedupon the type of polymerization and/or the type(s) of monomers utilized,but can generally include but are not limited to amines, peroxides, ormixtures thereof.

On the other hand, if the “grafting-to” method was utilized to attachthe first polymeric chain to the first anchoring compound via attachinga pre-formed first polymeric chain, the first polymeric chain can bedeactivated after covalently bonding the first polymeric chain to thefirst anchoring compound and prior to attaching the second anchoringcompound to the nanoparticle. Alternatively, the first polymeric chaincan be deactivated prior to covalently bonding the first polymeric chainto the first anchoring compound.

The deactivation of the first polymeric chain can be achieved by anysuitable process. In one embodiment, the polymer chain can be cleaved.Alternatively, the end of the polymer chain can be deactivated. Forexample, when formed via RAFT polymerization, the types of reactionsthat can be used to convert RAFT agents to deactivated end groupsinclude reactions with diazo compounds, reactions with nucleophilicreagents such as primary amines, and reactions with oxidation agentswhich cleave the RAFT agent off the chain end and form an oxidizedsulfur group such as sulfonic acid.

5. Attaching a Second Anchoring Compound to the Nanoparticle:

After attachment and deactivation of the first polymeric chain to thenanoparticle, a second anchoring compound can be attached to theremaining surface defined on the nanoparticle. This second anchoringcompound can be attached via any of the methods described above withrespect to the first anchoring compound. The second anchoring compoundand/or method of its attachment need not be the same as the firstanchoring compound. However, in one particular embodiment, the firstanchoring compound and the second anchoring compound are the same.

6. Formation of a Second Polymeric Chain Extending from theNanoparticle:

The second polymeric chain can be attached to the second anchoringcompound on the nanoparticle via the “grafting-from” method describedabove with respect to the first polymeric chain. The type(s) of monomersand/or polymerization technique for the formation of the secondpolymeric chain can be selected independently of the type of firstpolymeric chain already present on the nanoparticle. However, withoutwishing to be bound by any particular theory, it is presently believedthat the use of a “grafting-to” method, which would utilize a pre-formedsecond polymeric chain, may not be suitable due to the limited access ofsuch a pre-formed polymeric chain to the second anchoring agent on thesurface of the nanoparticle between the first polymeric chains.

7. Additional Polymeric Chains

Additional polymeric chains (e.g., a third polymeric chain, fourthpolymeric chain, etc.) can be attached to the nanoparticle as desiredfollowing the description above with respect to the attachment of thesecond polymeric chain.

8. Nanoparticles with Multiple Polymeric Assemblies:

According to these methods, nanoparticles with multiple polymericassemblies can be formed that have a first polymeric chain covalentlybonded to its surface via a first anchoring compound and a secondpolymeric chain covalently bonded to its surface via a second anchoringcompound. As stated, additional polymeric chains (e.g., a thirdpolymeric chain) can be further attached to the nanoparticles.

As used herein, the term “first polymeric chain” is meant to describe afirst type of polymeric chain, and one of ordinary skill in the artwould recognize that a multiple first polymeric chains could be presenton the nanoparticle (i.e., a first plurality of first polymeric chains).Likewise, the term “second polymeric chain” is meant to describe asecond type of polymeric chain, and one of ordinary skill in the artwould recognize that a multiple second polymeric chains could be presenton the nanoparticle (i.e., a second plurality of second polymericchains). Even further, the term “third polymeric chain” is meant todescribe a third type of polymeric chain, and one of ordinary skill inthe art would recognize that a multiple third polymeric chains could bepresent on the nanoparticle (i.e., a third plurality of third polymericchains).

As stated, the first polymeric chain can be different than the secondpolymeric chain (e.g., the polymeric first polymeric chain can have adifferent polydispersity index, molecular weight, etc. than the secondpolymeric chain). For instance, in one embodiment, the first polymericchain can have a molecular weight up to 50,000 (e.g., up to 25,000, upto 10,000, or about 500 to about 50,000), and the second polymeric chaincan have a molecular weight of about 50,000 or more. The use of such arelatively small molecular weight for the first polymeric chain can helpensure access to the remaining surface defined on the nanoparticle forattachment of the second anchoring compound.

In one embodiment, more first polymeric chains can be attached to thesurface of the nanoparticle than second polymeric chains.

Polymerization Techniques

As stated, the first and second polymeric chains can be formed viacontrolled polymerizations, such as controlled living polymerizations(CLPs) or controlled ring-opening polymerizations, which may beindependently selected for each of the first and second polymeric chainsbased upon the particular anchoring agent present on the nanoparticle,type of monomer(s) used to form the polymeric chain, and/or desiredproperties of the polymeric chains formed. Through the use of thesecontrolled polymerizations, each polymeric chain can be produced withlow polydispersity and diverse architectures. Thus, these methods areideal for block polymer and/or graft polymer synthesis.

Controlled living polymerization generally refers to chain growthpolymerization which proceeds with significantly suppressed terminationor chain transfer steps. Thus, polymerization in CLP proceeds until allmonomer units have been consumed or until the reaction is terminated(e.g., through quenching and/or deactivating), and the addition ofmonomer results in continued polymerization, making CLP ideal for blockpolymer and graft polymer synthesis. The molecular weight of theresulting polymer is generally a linear function of conversion so thatthe polymeric chains are initiated and grow substantially uniformly.Thus, CLPs provide precise control on molecular structures,functionality and compositions. Thus, these polymers can be tuned withdesirable compositions and architectures.

Controlled living polymerizations can be used to produce blockcopolymers because CLP can leave a functional terminal group on thepolymer formed (e.g., a halogen functional group). For example, in thecopolymerization of two monomers (A and B) allowing A to polymerize viaCLP will exhaust the monomer in solution with minimal termination. Aftermonomer A is fully reacted, the addition of monomer B will result in ablock copolymer. Controlled ring-opening polymerizations can utilizesuitable catalysts such as tin(II) to open the rings of monomers to forma polymer.

Several of such polymerization techniques are discussed in thisapplication. These techniques are generally known to those skilled inthe art. A brief general description of each technique is below, and isprovided for further understanding of the present invention, and is notintended to be limiting:

A. Reversible Addition-Fragmentation Chain Transfer Polymerization

Reversible Addition-Fragmentation chain Transfer polymerization (RAFT)is one type of controlled radical polymerization. RAFT polymerizationuses thiocarbonylthio compounds, such as dithioesters, dithiocarbamates,trithiocarbonates, and xanthates, in order to mediate the polymerizationvia a reversible chain-transfer process. RAFT polymerization can beperformed by simply adding a chosen quantity of appropriate RAFT agents(thiocarbonylthio compounds) to a conventional free radicalpolymerization. RAFT polymerization is particularly useful with monomershaving a vinyl functional group (e.g., a (meth)acrylate group).

Typically, a RAFT polymerization system includes the monomer, aninitiator, and a RAFT agent (also referred to as a chain transferagent). Because of the low concentration of the RAFT agent in thesystem, the concentration of the initiator is usually lower than inconventional radical polymerization. Suitable radical initiators can beazobisisobutyronitrile (AIBN), 4,4′-azobis(4-cyanovaleric acid) (ACVA),etc.

RAFT agents are generally thiocarbonylthio compounds, such as generallyshown below:

where the z group primarily stabilizes radical species added to the C═Sbond and the R group is a good homolytic leaving group which is able toinitiate monomers. For example, the z group can be an aryl group (e.g.,phenyl group, benzyl group, etc.), an alkyl group, etc. The R″ group canbe an organic chain terminating with a carboxylic acid group.

As stated, RAFT is a type of living polymerization involving aconventional radical polymerization in the presence of a reversiblechain transfer reagent. Like other living radical polymerizations, thereis minimized termination step in the RAFT process. The reaction isstarted by radical initiators (e.g., AIBN). In this initiation step, theinitiator reacts with a monomer unit to create a radical species whichstarts an active polymerizing chain. Then, the active chain reacts withthe thiocarbonylthio compound, which kicks out the homolytic leavinggroup (R″). This is a reversible step, with an intermediate speciescapable of losing either the leaving group (R″) or the active species.The leaving group radical then reacts with another monomer species,starting another active polymer chain. This active chain is then able togo through the addition-fragmentation or equilibration steps. Theequilibration keeps the majority of the active propagating species intothe dormant thiocarbonyl compound, limiting the possibility of chaintermination. Thus, active polymer chains are in equilibrium between theactive and dormant species. While one polymer chain is in the dormantstage (bound to the thiocarbonyl compound), the other is active inpolymerization.

By controlling the concentration of initiator and thiocarbonylthiocompound and/or the ratio of monomer to thiocarbonylthio compound, themolecular weight of the polymeric chains can be controlled with lowpolydispersities.

Depending on the target molecular weight of final polymers, the monomerto RAFT agent ratios can range from about less than about 10 to morethan about 1000 (e.g., about 10 to about 1,000). Other reactionparameters can be varied to control the molecular weight of the finalpolymers, such as solvent selection, reaction temperature, and reactiontime. For instance, solvents can include conventional organic solventssuch as tetrahydrofuran, toluene, dimethylformamide, anisole,acetonitrile, dichloromethane, etc. The reaction temperature can rangefrom room temperature (e.g., about 20° C.) to about 120° C. The reactiontime can be from less than about 1 h to about 48 h.

The RAFT process allows the synthesis of polymers with specificmacromolecular architectures such as block, gradient, statistical,comb/brush, star, hyperbranched, and network copolymers.

Because RAFT polymerization is a form of living radical polymerization,it is ideal for synthesis of block copolymers. For example, in thecopolymerization of two monomers (A and B) allowing A to polymerize viaRAFT will exhaust the monomer in solution with significantly suppressedtermination. After monomer A is fully reacted, the addition of monomer Bwill result in a block copolymer. One requirement for maintaining anarrow polydispersity in this type of copolymer is to have a chaintransfer agent with a high transfer constant to the subsequent monomer(monomer B in the example).

Using a multifunctional RAFT agent can result in the formation of a starcopolymer. RAFT differs from other forms of CLPs because the core of thecopolymer can be introduced by functionalization of either the R groupor the Z group. While utilizing the R group results in similarstructures found using ATRP or NMP, the use of the Z group makes RAFTunique. When the Z group is used, the reactive polymeric arms aredetached from the core while they grow and react back into the core forthe chain-transfer reaction.

B. Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is another example of aliving radical polymerization. The control is achieved through anactivation-deactivation process, in which most of the reaction speciesare in dormant format, thus significantly reducing chain terminationreaction. The four major components of ATRP include the monomer,initiator, ligand, and catalyst. ATRP is particularly useful monomershaving a vinyl functional group (e.g., a (meth)acrylate group).

Organic halides are particularly suitable initiators, such as alkylhalides (e.g., alkyl bromides, alkyl chlorides, etc.). For instance, inone particular embodiment, the alkyl halide can be ethyl2-bromoisobutyrate. The shape or structure of the initiator can alsodetermine the architecture of the resulting polymer. For example,initiators with multiple alkyl halide groups on a single core can leadto a star-like polymer shape.

The catalyst can determine the equilibrium constant between the activeand dormant species during polymerization, leading to control of thepolymerization rate and the equilibrium constant. In one particularembodiment, the catalyst is a metal having two accessible oxidationstates that are separated by one electron, and a reasonable affinity forhalogens. One particularly suitable metal catalyst for ATRP is copper(I).

The ligands can be linear amines or pyridine-based amines.

Depending on the target molecular weight of final polymers, the monomerto initiator ratios can range from less than about 10 to more than about1,000 (e.g., about 10 to about 1,000). Other reaction parameters can bevaried to control the molecular weight of the final polymers, such assolvent selection, reaction temperature, and reaction time. Forinstance, solvents can include conventional organic solvents such astetrahydrofuran, toluene, dimethylformamide, anisole, acetonitrile,dichloromethane, etc. The reaction temperature can range from roomtemperature (e.g., about 20° C.) to about 120° C. The reaction time canbe from less than about 1 h to about 48 h.

C. Nitroxide-Mediated Polymerization

Nitroxide-mediated polymerization (NMP) is another form of controlledliving polymerization utilizing a nitroxide radical, such as shownbelow:

where R1 and R2 are, independently, organic groups (e.g., aryl groupssuch as phenyl groups, benzyl groups, etc.; alkyl groups, etc.). NMP isparticularly useful with monomers having a vinyl functional group (e.g.,a (meth)acrylate group).

D. Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROMP) is a type of olefinmetathesis polymerization. The driving force of the reaction is reliefof ring strain in cyclic olefins (e.g. norbornene or cyclopentene) inthe presence of a catalyst. The catalysts used in a ROMP reaction caninclude a wide variety of metals and range from a simple RuCl₃/alcoholmixture to Grubbs' catalyst.

In this embodiment, the monomer can include a strained ring functionalgroup, such as a norbornene functional group, a cyclopentene functionalgroup, etc. to form the polymeric chains. For example, norbornene is abridged cyclic hydrocarbon that has a cyclohexene ring bridged with amethylene group in the para position.

The ROMP catalytic cycle generally requires a strained cyclic structurebecause the driving force of the reaction is relief of ring strain.After formation of the metal-carbene species, the carbene attacks thedouble bond in the ring structure forming a highly strainedmetallacyclobutane intermediate. The ring then opens giving thebeginning of the polymer: a linear chain double bonded to the metal witha terminal double bond as well. The new carbene reacts with the doublebond on the next monomer, thus propagating the reaction.

E. Ring-Opening Polymerization

In one particular embodiment, where the monomer includes a strained ringfunction group (e.g., a caprolactone or lactide), ring-openingpolymerization (ROP) may be used to form the polymeric chain. Forexample, a caprolcatone-substituted monomer is a polymerizable ester,which can undergo polymerization with the aid of an alcohol as aninitiator and a tin-based reagent as a catalyst.

Examples Materials

Unless otherwise specified, all chemicals were purchased from FisherScientific and used as received. Colloidal silica particles (15 nmdiameter) were purchased from Nissan Chemical. 2,2′-Azoisobutyronitrile(AIBN) was used after recrystallization in ethanol. Styrene and methylmethacrylate were passed through a basic alumina column to remove theinhibitor before use. Activated 4-cyanopentanoic acid dithiobenzoate(CPDB) was prepared according to a procedure described in literature.⁷3-Aminopropyldimethylethoxysilane, dimethylmethoxy-n-octylsilane and3-trimethoxysilylpropyl-2-bromo-2-methylpropionate were purchased fromGelest Inc and used as received.

Instrumentation:

NMR spectra were recorded on a Varian 300 spectrometer using CDCl₃ assolvent. Molecular weights and molecular weight distributions weredetermined using a Waters gel-permeation chromatograph equipped with a515 HPLC pump, 2410 refractive index detector, three Styragel columns(HR1, HR3, HR4 in the effective molecular weight range of 100-5000,500-30000 and 5000-500000, respectively) with THF as eluent at 30° C.and a flow rate of 1.0 mL/min. The GPC system was calibrated withpoly(methyl methacrylate) and polystyrene standards obtained fromPolymer Labs.

-   1. Synthesis of CPDB Anchored Silica Nanoparticles:

A solution (10 ml) of colloidal silica particles (30 wt % in MIBK) wasadded to a two necked round-bottom flask and diluted with 75 ml of THF.To it was added 3-aminopropyldimethylethoxysilane (0.16 ml, 1 mmol) andthe mixture was refluxed at 75° C. overnight under nitrogen protection.The reaction was then cooled to room temperature and precipitated inlarge amount of hexanes. The particles were then recovered bycentrifugation and dispersed in THF using sonication and precipitated inhexanes again. The amino functionalized particles were then dispersed in40 ml of THF for further reaction.

A THF solution of the amino functionalized silica nanoparticles (40 ml,1.8 g) was added drop wise to a THF solution (30 ml) of activated CPDB(0.25 g, 0.65 mmol) at room temperature. After complete addition, thesolution was stirred overnight. The reaction mixture was thenprecipitated into a large amount of 4:1 mixture of cyclohexane and ethylether (2500 ml). The particles were recovered by centrifugation at 3000rpm for 8 minutes. The particles were then re-dispersed in 30 ml THF andprecipitated in 4:1 mixture of cyclohexane and ethyl ether. Thisdissolution-precipitation procedure was repeated 2 more times until thesupernatant layer after centrifugation was colorless. The red CPDBanchored silica nanoparticles were dried at room temperature andanalyzed using UV analysis for the chain density.

Several such CPDB anchored silica nanoparticles having differentgrafting density from 0.05 to 0.6 were prepared by adjusting the ratioof the 3-aminopropyldimethylethoxysilane to colloidal silicananoparticles.

-   2. Synthesis of Bimodal Silica Grafted Polymethylmethacrylate (PMMA)    Nanoparticles by Step-by-Step RAFT Polymerization:

A. Graft Polymerization of Methyl Methacrylate Monomer from CPDBAnchored Colloidal Silica Nanoparticles to Graft 1^(st) Chain fromSurface of Nanoparticles:

A solution of methyl methacrylate (7 mL), CPDB anchored silicananoparticles (300 mg, 80 μmol/g), AIBN (2.40 μmol), and THF (7 mL) wasprepared in a dried Schlenk tube. The mixture was degassed by threefreeze-pump-thaw cycles, back filled with nitrogen, and then placed inan oil bath at 60° C. for 3 h. The polymerization solution was quenchedin ice water and poured into cold methanol to precipitate polymergrafted silica nanoparticles. The polymer chains were cleaved bytreating a small amount of nanoparticles with HF and the resultingpolymer chains were analyzed by GPC. The polymer cleaved from theSi-g-PMMA particles had a molecular weight of 24400 g/mol and PDI of1.07.

B. Cleavage of RAFT Agent from 1^(st) Brush:

Solid AIBN (24 μmol) was added to a solution of Si-g-PMMA in THF (0.4 gin 20 ml) and heated at 65° C. under nitrogen for 30 minutes. Theresulting white solution mixture was poured into 100 ml hexanes andcentrifuged at 8000 rpm for 5 minutes to recover Si-g-PMMAnanoparticles.

C. Functionalization of Si-g-PMMA by 2^(nd) RAFT Agent:

The second RAFT agent was attached onto the surface of the silica whichwas not covered by the first polymer chain. The bare surface of thenanoparticles was functionalized by amine groups using 0.01 ml of3-aminopropyldimethylethoxysilane in a process similar to the first RAFTagent attachment. The second RAFT agent was attached by reaction 30 mgof activated CPDB (0.030 g) at room temperature with theamino-functional particles.

D. Graft Polymerization of Methyl Methacrylate from Si-g-PMMA toSynthesize 2^(nd) Brush:

The CPDB anchored Si-g-PMMA particles (0.4 g) dissolved in 10 mL THFwere added to a dried Schlenk tube along with 15 ml MMA and AIBN (45 μlof 0.005M THF solution). The mixture was degassed by threefreeze-pump-thaw cycles, back filled with nitrogen, and then placed inan oil bath at 65° C. for 12 hours. The polymerization was quenched inice water. The polymer was recovered by precipitating into hexane andcentrifugation at 8000 rpm. GPC results indicated the 2^(nd) chain has amolecular weight of 103000 g/mol and PDI of 1.13.

-   3. Synthesis of Bimodal Silica Grafted Polystyrene (PS)    Nanoparticles by Step-by-Step RAFT Polymerization

A. Graft Polymerization of Styrene from CPDB Anchored Colloidal SilicaNanoparticles to Graft 1^(st) Chain from Surface of Nanoparticles:

A solution of styrene (25 mL), CPDB anchored silica nanoparticles (1.4g, 80 μmol/g), AIBN (1.8 ml, 5 mM solution in THF), and THF (25 mL) wasprepared in a dried Schlenk tube. The mixture was degassed by threefreeze-pump-thaw cycles, back filled with nitrogen, and then placed inan oil bath at 65° C. for 4 hours. The polymerization solution wasquenched in ice water and poured into cold methanol to precipitatepolymer grafted silica nanoparticles. The polymer chains were cleaved bytreating a small amount of nanoparticles with HF and the resultingpolymer chains were analyzed by GPC. The polymer cleaved from theSi-g-PS particles had a molecular weight of 1600 g/mol and PDI of 1.26.

B. Cleavage of RAFT Agent from 1^(st) Brush:

Solid AIBN (250 mg) was added to a solution of Si-g-PS in THF (2 g in 50ml) and heated at 65° C. under nitrogen for 30 minutes. The resultingwhite solution mixture was poured into 200 ml hexanes and centrifuged at8000 rpm for 5 minutes to recover Si-g-PS nanoparticles.

C. Functionalization of Si-g-PS by 2^(st) RAFT agent:

The second RAFT agent was attached onto the surface of the silica whichwas not covered by the first polymer chain. The bare surface of thenanoparticles was functionalized by amine groups using 0.01 ml of3-aminopropyldimethylethoxysilane in a process similar to the first RAFTagent attachment. The second RAFT agent was attached by reaction 30 mgof activated CPDB (0.030 g) at room temperature with theamino-functional particles.

D. Graft Polymerization of Styrene from Si-g-PS to Synthesize 2^(st)Brush:

The CPDB anchored Si-g-PS particles (1.4 g by weight of bare silica)dissolved in 10 mL THF were added to a dried Schlenk tube along with 20ml styrene and AIBN (1.8 mL of 0.005M THF solution). The mixture wasdegassed by three freeze-pump-thaw cycles, back filled with nitrogen,and then placed in an oil bath at 65° C. for 18 hours. Thepolymerization was quenched in ice water. The polymer was recovered byprecipitating into hexane and centrifugation at 8000 rpm. GPC resultsindicated the 2^(nd) chain has a molecular weight of 40,000 g/mol andPDI of 1.19.

-   4. Synthesis of Mixed Brush of Polystyrene and    Polymethylmethacrylate (PMMA) Grafted Silica Nanoparticles by    Step-by-Step RAFT Polymerization

A. Graft Polymerization of Styrene from CPDB Anchored Colloidal SilicaNanoparticles to Graft 1^(st) Chain from Surface of Nanoparticles:

A solution of styrene (10 mL), CPDB anchored silica nanoparticles (0.5g, 80 μmol/g), AIBN (0.600 ml, 5 mM solution in THF), and THF (10 mL)was prepared in a dried Schlenk tube. The mixture was degassed by threefreeze-pump-thaw cycles, back filled with nitrogen, and then placed inan oil bath at 65° C. for 4 hours. The polymerization solution wasquenched in ice water and poured into cold methanol to precipitatepolymer grafted silica nanoparticles. The polymer chains were cleaved bytreating a small amount of nanoparticles with HF and the resultingpolymer chains were analyzed by GPC. The polymer cleaved from theSi-g-PS particles had a molecular weight of 5000 g/mol and PDI of 1.13.

B. Cleavage of RAFT Agent from 1^(st) Brush:

Solid AIBN (108 mg) was added to a solution of Si-g-PS in THF (0.5 g in50 ml) and heated at 65° C. under nitrogen for 30 minutes. The resultingwhite solution mixture was poured into 200 ml hexanes and centrifuged at8000 rpm for 5 minutes to recover Si-g-PS nanoparticles.

C. Functionalization of Si-g-PS by 2^(nd) RAFT Agent:

The second RAFT agent was attached onto the surface of the silica whichwas not covered by the first polymer chain. The bare surface of thenanoparticles was functionalized by amine groups using 0.0025 ml of3-aminopropyldimethylethoxysilane in a process similar to the first RAFTagent attachment. The second RAFT agent was attached by reaction 30 mgof activated CPDB (0.030 g) at room temperature with theamino-functional particles.

D. Graft Polymerization of Methyl Methacrylate from Si-g-PS toSynthesize 2^(nd) Brush:

The CPDB anchored Si-g-PS particles (0.5 g by weight of bare silica)dissolved in 10 mL THF were added to a dried Schlenk tube along with 20ml styrene and AIBN (0.01 ml of 0.005M THF solution). The mixture wasdegassed by three freeze-pump-thaw cycles, back filled with nitrogen,and then placed in an oil bath at 60° C. for 14 hours. Thepolymerization was quenched in ice water. The polymer was recovered byprecipitating into hexane and centrifugation at 8000 rpm. GPC resultsindicated the 2^(nd) chain has a molecular weight of 205,000 g/mol andPDI of 1.17.

-   5. Synthesis of Mixed Brush of Polymethyl Methacrylate and    Poly(t-Butyl Methacrylate) Grafted Silica Nanoparticles by    Step-by-Step RAFT Polymerization

A. Graft Polymerization of Methyl Methacrylate from CPDB AnchoredColloidal Silica Nanoparticles to Graft 1^(st) Chain from Surface ofNanoparticles:

A solution of methyl methacrylate (10 mL), CPDB anchored silicananoparticles (0.5 g, 80 μmol/g), AIBN (0.600 ml, 5 mM solution in THF),and THF (10 mL) was prepared in a dried Schlenk tube. The mixture wasdegassed by three freeze-pump-thaw cycles, back filled with nitrogen,and then placed in an oil bath at 60° C. for 3 hours. The polymerizationsolution was quenched in ice water and poured into cold methanol toprecipitate polymer grafted silica nanoparticles. The polymer chainswere cleaved by treating a small amount of nanoparticles with HF and theresulting polymer chains were analyzed by GPC. The polymer cleaved fromthe Si-g-PMMA particles had a molecular weight of 5000 g/mol and PDI of1.17.

B. Cleavage of RAFT Agent from 1^(st) Brush:

Solid AIBN (108 mg) was added to a solution of Si-g-PMMA in THF (0.5 gin 50 ml) and heated at 65° C. under nitrogen for 30 minutes. Theresulting white solution mixture was poured into 100 ml hexanes andcentrifuged at 8000 rpm for 5 minutes to recover Si-g-PMMAnanoparticles.

C. Functionalization of Si-g-PS by 2^(nd) RAFT Agent:

The second RAFT agent was attached onto the surface of the silica whichwas not covered by the first polymer chain. The bare surface of thenanoparticles was functionalized by amine groups using 0.0025 ml of3-aminopropyldimethylethoxysilane in a process similar to the first RAFTagent attachment. The second RAFT agent was attached by reaction 30 mgof activated CPDB (0.030 g) at room temperature with theamino-functional particles.

D. Graft Polymerization of t-Butyl Methacrylate from Si-g-PMMA toSynthesize 2^(nd) Brush:

The CPDB anchored Si-g-PMMA particles (0.105 g) dissolved in 7 mL THFwere added to a dried Schlenk tube along with 0.500 ml t-butylmethacrylate and AIBN (10 μl of 0.005M THF solution). The mixture wasdegassed by three freeze-pump-thaw cycles, back filled with nitrogen,and then placed in an oil bath at 65° C. for 12 hours. Thepolymerization was quenched in ice water. The polymer was recovered byprecipitating into hexane and centrifugation at 8000 rpm. GPC resultsindicated the 2^(nd) chain has a molecular weight of 17000 g/mol and PDIof 1.24.

-   6. Synthesis of Bimodal Polystyrene Brush Grafted Silica    Nanoparticles by Step-by-Step RAFT and ATRP Polymerization

A. Graft Polymerization of Styrene from CPDB Anchored Colloidal SilicaNanoparticles to Graft 1^(st) Chain from Surface of Nanoparticles:

A solution of methyl methacrylate (10 mL), CPDB anchored silicananoparticles (0.3 g, 80 μmol/g), AIBN (0.240 ml, 5 mM solution in THF),and THF (10 mL) was prepared in a dried Schlenk tube. The mixture wasdegassed by three freeze-pump-thaw cycles, back filled with nitrogen,and then placed in an oil bath at 65° C. for 4 hours. The polymerizationsolution was quenched in ice water and poured into cold methanol toprecipitate polymer grafted silica nanoparticles. The polymer chainswere cleaved by treating a small amount of nanoparticles with HF and theresulting polymer chains were analyzed by GPC. The polymer cleaved fromthe Si-g-PMMA particles had a molecular weight of 10400 g/mol and PDI of1.12.

B. Cleavage of RAFT Agent from 1^(st) Brush:

Solid AIBN (110 mg) was added to a solution of Si-g-PS in THF (0.5 g in50 ml) and heated at 65° C. under nitrogen for 30 minutes. The resultingwhite solution mixture was poured into 100 ml hexanes and centrifuged at8000 rpm for 5 minutes to recover Si-g-PS nanoparticles.

C. Functionalization of Si-g-PS by ATRP Initiator Agent:

The ATRP initiator was attached onto the surface of the silica which wasnot covered by the first polymer chain. A solution (0.3 g by weight ofsilica) of Si-g-PS was added to a two necked round-bottom flask anddiluted with 25 ml of THF. To it was added 0.025 ml of3-trimethoxysilylpropyl-2-bromo-2-methylpropionate and the mixture wasrefluxed at 75° C. overnight under nitrogen protection. The reaction wasthen cooled to room temperature and precipitated in large amount ofhexanes. The particles were then recovered by centrifugation anddispersed in THF using sonication and precipitated in hexanes again. TheATRP initiator functionalized particles were then dispersed in 10 ml ofTHF for further reaction.

D. ATRP Polymerization of Styrene from Si-g-PS to Synthesize 2^(nd) PSBrush:

The styrene monomer (10 ml), Cu(I)Cl (0.189 mmol) and Me₆Tren ligand(0.38 mmol) was added to a Schlenk flask and degassed by purgingnitrogen for 10 minutes. In another flask ATRP initiator anchoredSi-g-PS particles (0.3 g by weight of silica) were dissolved in 10 mLTHF and the solution was degassed using nitrogen for 10 minutes. Thenanoparticle solution was then added to the Schlenk flask and theSchlenk flask was then placed in an oil bath at 90° C. for 36 hours. Thepolymerization was quenched in ice water. The polymer was recovered byprecipitating into methanol and centrifugation at 8000 rpm, followed byredispersion in THF. The process was repeated 4 more times to remove thecopper catalyst. GPC results indicated the 2^(nd) chain has a molecularweight of 255000 g/mol and PDI of 1.43.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood the aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in the appended claims.

What is claimed:
 1. A nanoparticle with multiple polymeric assemblies,comprising: a nanoparticle defining a surface; a first polymeric chaincovalently bonded to the surface of the nanoparticle; and a secondpolymeric chain covalently bonded to the surface of the nanoparticle,wherein the first polymeric chain is different than the second polymericchain.
 2. The nanoparticle as in claim 1, wherein the first polymericchain is covalently bonded to the nanoparticle via a first anchoringcompound, and wherein the second polymeric chain is covalently bonded tothe nanoparticle via a second anchoring compound.
 3. The nanoparticle asin claim 1, wherein the first polymeric chain has a molecular weight upto about 50,000.
 4. The nanoparticle as in claim 1, wherein the secondpolymeric chain has a molecular weight of about 50,000 or more.
 5. Thenanoparticle as in claim 1, wherein the first polymeric chain has adifferent polydispersity index than the second polymeric chain.
 6. Thenanoparticle as in claim 1, wherein the first polymeric chain comprisesa single type of monomer.
 7. The nanoparticle as in claim 1, wherein thefirst polymeric chain comprises a mixture of different types ofmonomers.
 8. The nanoparticle as in claim 1, wherein the secondpolymeric chain comprises a single type of monomer.
 9. The nanoparticleas in claim 1, wherein the second polymeric chain comprises a mixture ofdifferent types of monomers.
 10. The nanoparticle as in claim 1, furthercomprising: a third polymeric chain covalently bonded to the surface ofthe nanoparticle, wherein the third polymeric chain is different thanthe first polymeric chain and the second polymeric chain.
 11. Thenanoparticle as in claim 10, further comprising: a fourth polymericchain covalently bonded to the surface of the nanoparticle, wherein thefourth polymeric chain is different than the first polymeric chain, thesecond polymeric chain, and the third polymeric chain.
 12. Thenanoparticle as in claim 1, wherein the nanoparticle comprises a naturalor synthetic nanoclay.
 13. The nanoparticle as in claim 1, wherein thenanoparticle comprises an inorganic metal oxide.
 14. The nanoparticle asin claim 1, wherein the nanoparticle is an inorganic nanoparticle. 15.The nanoparticle as in claim 15, wherein the inorganic nanoparticlecomprises silica, alumina, titania, indium tin oxide, CdSe, or mixturesthereof.
 16. The nanoparticle as in claim 1, wherein the nanoparticlecomprises an organic nanoparticle.
 17. The nanoparticle as in claim 1,wherein the organic nanoparticle comprises a polymer, carbon, graphite,graphene, carbon nanotubes, virus nanoparticles, or mixtures thereof.